A conventional rotation angle detecting device comprises a stator having a ring-shaped stator core and a rotor which is made of a magnetic material and is disposed in the stator, wherein the stator is provided with pluralities of salient poles directed in a centripetal direction from the ring-shaped stator core, and wherein a coil is wound around each of the salient poles (please see Patent Literature 1 or 2).
However, such a conventional rotation angle detecting device has a large device thickness in an axial direction of the rotor because the coil is wound around each of the salient poles directed perpendicularly to the axial direction of the rotor.
For the purpose of reducing thickness of a whole rotation angle detecting device, another conventional rotation angle detecting device has a constitution that a sheet coil is applied as each of an excitation coil disposed on a base portion of a stator and a detecting coil disposed on a rotating plate fixed to a rotating shaft of a rotor so as to face the excitation coil, and that an output transformer is constructed from a primary coil disposed on an outer peripheral surface of the rotor and a secondary coil disposed on an inner peripheral surface of the base portion of the stator so as to face the primary coil (please see Patent Literature 3).
However, there is a problem with the device with which such a sheet coil is used that an outer diameter of the whole rotation angle detecting device centered on the rotating shaft is large because the primary and secondary coils constructing the output transformer are respectively disposed on the outer peripheral surface of the rotor and the inner peripheral surface of the base portion of the stator, and the excitation coil and the detecting coil are further arranged at an outside of the output transformer so as to face each other.
The present inventor provided a device for the purpose of reducing thickness of a whole rotation angle detecting device, wherein the device has a constitution that a sheet coil is applied as each of the primary and secondary coils constructing the output transformer in the Patent Literature 3, and that each of these sheet coils are disposed on each of a base portion of a stator and a facing surface of a rotating plate opposing to the base portion at an inside of an excitation coil and a detecting coil each of which consists of respective other sheet coils (please see PCT/JP2015/82047 and the application entered into the national phase in Japan: Patent Application No. 2016-509209).